The casting of the solution of resin dispersion is a conventional method of making a micrometer-scale freestanding membrane. However, it is challenging for this conventional method to produce an ultra-thin freestanding membrane in nanometer scale due to the difficulty in handling such ultra-thin material. The art lacks a solution to the handling problem and lacks the ability for producing a freestanding recast membrane having thickness in nanometer scale over an ultra-thin nanoporous substrate.
The pore-filling method is the prior technology used to make freestanding membranes by filling the pores of a substrate with polymer. This technology relies on the porous substrate with specific pore size and its uniformity to guarantee the complete filling of all pores. Therefore, the thickness of polymeric membrane is dictated by the thickness of the substrate, which is usually thick in order to accommodate large pores. However, the thickness also depends on the polymer used.
It would seem obvious to another researcher that spin coating would be a preferred method to casting when making a uniform nanometer-scale film from the solution of resin dispersion. To make a freestanding membrane, the thin film would need to be transferred to a porous support; otherwise, the solution would need to be spin-coated on a porous substrate directly. Since the substrate cannot be too thick, it is more likely that the porous substrate would not survive the spin coating process. In addition, the polymer might get into the pores and across to the other side, thus resulting in a thicker membrane if a thicker substrate is used, or even worse, leaving pin holes in the membrane.
Conventionally ultra-thin nanometer-scale freestanding cross-link membranes are synthesized by a method called the interfacial polymerization, which is processed over a thick and sturdy porous support resulting in polymerized membrane on one side of the support with partial intrusion into the pores.
Prior technology is based on stacking different types of support to enhance the strength of the membrane, while minimizing the resistance to the flow across the membrane by using supports with progressively larger pore size.